Two types of highly repeated DNA segments in the genome of the African green monkey (Cercopithecus aethiops) are being studied. 1) Satellite DNAs are characterized by tandem repetitions and are localized to centromeres. We investigated the inheritance of the polymorphic forms of deca-satellite that are found in individual monkeys of the species. Although the fluid organization of the monkey deca-satellite was confirmed, the analysis did not afford any direct evidence for major satellite reorganization in a single generation. However in tissue culture cells, in situ hybridization of deca-satellite probes to endoreduplicated chromosomes indicated extensive unequal crossing-over of deca-satellite sequences among sister chromatides. 2) The members of the primate KpnI family of long interspersed repeated sequences were shown to be a) polymorphic in length and internal sequence arrangement, b) transcribed in human and monkey cells, and c) homologous to a similarly abundant rodent LINE family (about 104 copies per genome). A cloned member of the R family of murine short interspersed repeated sequences contains a promoter that supports gene expression in mammalian cells.